The present invention relates generally to a thin film capacitor, and more particularly to a thin film capacitor having amorphous and polycrystalline ferroelectric films serving as a dielectric material. In addition, the present invention also relates to a preparation of the amorphous ferroelectric film by sputtering.
In light of the miniaturization of the semiconductor element, and the high dielectric constant of the barium strontium titanate [hereinafter abbreviated as (Ba,Sr)TiO3] film, the (Ba,Sr)TiO3 film is used as a dielectric layer of a thin film capacitor of the integrated circuit. The magnitude of the leakage current of the thin film capacitor is directly proportional to the loss of the electric charge that is stored in the thin film capacitor. For this reason, a solution to bring about a reduction in the leakage current of the thin film capacitor is sought by the industry.
The (Ba,Sr)TiO3 film, which is prepared by sputtering at high temperature or at low temperature preceding a high temperature annealing, exists in the polycrystalline form. When the polycrystalline (Ba,Sr)TiO3 film is used as the dielectric layer of a thin film capacitor, the thin film capacitor has a tendency to exhibit a relatively large leakage current. In order to inhibit the leakage current, a thicker (Ba,Sr)TiO3 film may be used. Alternatively, a high temperature oxygen treatment may be carried out in a furnace tube after the formation of the electrodes of the thin film capacitor. However, the sputtering time must be prolonged in the former approach. The latter requires an additional high temperature process.
U.S. Pat. No. 5,471,364 discloses an electrode interface for use together with a material having high dielectric constant. In other words, a buffer layer having a low leakage current density is formed between the electrodes and the high dielectric constant material. In the embodiment of the above disclosure. (Ba,Sr)TiO3 is used as a high dielectric constant material, whereas SrTiO3 is used as the buffer layer. Even though the crystalline forms of (Ba,Sr)TiO3 and SrTiO3 are not explained in the patent specification, (Ba,Sr)TiO3 film and SrTiO3 film, which are formed by the sputtering method, always exist in the polycrystalline form.
The first objective of the present invention is to provide a method for forming an amorphous ferroelectric thin film by sputtering. The amorphous ferroelectric thin film continues to exist as the amorphous ferroelectric thin film in the wake of a high temperature annealing treatment.
The second objective of the present invention is to provide a thin film capacitor having an amorphous ferroelectric thin film/polycrystalline ferroelectric thin film.
The third objective of the present invention is to provide a process for making a thin film capacitor having an amorphous ferroelectric thin film/polycrystalline ferroelectric thin film.
The first objective of the present invention is attained by a method for forming an amorphous ferroelectric thin film by sputtering, as defined in claim 1 of the present application. The amorphous ferroelectric thin film may have a composition of (Ba,Sr,Pb,La)(Ti,Zr)O3, preferably, of (Ba,Sr)TiO3. In the wake of being treated by a high temperature annealing, the amorphous ferroelectric thin film formed by the method of the present invention continues to remain in the amorphous form. If the flow rate of the reactive gas in the method of claim 1, such as oxygen, was reduced to substantially zero, the ferroelectric thin film so formed was amorphous; however, the amorphous ferroelectric thin film was transformed into the polycrystalline form by a high temperature annealing treatment. Or under the circumstance that the flow rate of the reactive gas (oxygen) was lowered to substantially zero, and that the substrate temperature was raised to a high temperature, such as  greater than 400xc2x0 C., the ferroelectric thin film formed by the method of the present invention was in the polycrystalline form. It was found that the crystallinity of the ferroelectric thin film improved in response to each increment in the substrate temperature. The phenomenon of the amorphous ferroelectric thin film formed by the method of the present invention continuing to remain in the amorphous form as described above is not completely understood by this inventor of the present invention. It is a presumption of this inventor that the energy of the sputtering source might have been dispersed by the reactive gas which was introduced into the process.
The thin film capacitor of the second objective of the present invention and the method for forming the thin film capacitor of the third objective of the present invention are respectively defined in claim 8 and claim 15 of the present application. According to the present invention, the leakage current of the polycrystalline ferroelectric thin film (used as a dielectric layer) is inhibited when the amorphous ferroelectric thin film is used as a buffer layer of the thin film capacitor.
Preferably, only at an interface between the bottom metal electrode and the dielectric layer of the thin film capacitor is provided with the buffer layer.
Apparently, an add-on amorphous ferroelectric thin film serving as an add-on buffer layer may be provided at an interface between the polycrystalline ferroelectric thin film and an upper electrode of the thin film capacitor of the present invention. The add-on buffer layer is intended to inhibit further the leakage current of the thin film capacitor. The amorphous and the polycrystalline ferroelectric thin films can independently have the composition of (Ba,Sr,Pb,La)(Ti,Zr)O3, preferably, (Ba,Sr)TiO3. Preferably, the amorphous and the polycrystalline ferroelectric thin films have substantially the same composition.